1. Technical Field of the Invention:
The present invention relates generally to interface circuitry for low voltage digital technologies and in particular to output drivers. Still more particularly, the present invention a mixed voltage output driver with automatic impedance adjustment.
2. Description of the Related Art:
Efforts to decrease the size, increase the speed and reduce the power consumption of electrical circuits have created the need for low voltage silicon construction. Lower voltage requirements result in lower power consumption which complements, e.g., battery powered, portable electronics.
With advances in semiconductor fabrication techniques, the size of electronic devices has been reduced to the sub-micron level and the voltage requirements of these devices have been reduced significantly. Nevertheless, when a new low-voltage integrated circuit (IC) technology is developed, it is often desirable for that new technology to be able to operate with existing relatively high-voltage circuitry. The voltage of a particular technology is typically defined by the gate-oxide breakdown voltage and/or the punch-through between the source and drain.
As a result of the differing IC technologies, there are occasions when interfaces between "chips" occur where different chips drive different uplevel, i.e., logical high, voltages. For example, one driver circuit may drive the wire, i.e., interconnection, to an uplevel voltage of 2.0V. After this driver has driven the wire, it could tristate, i.e., go into a high impedance mode, relinquishing its turn on the wire. At this time, a second driver circuit on the wire may drive the wire to a 1.5V uplevel. In this example, the receivers on the wire would probably have logic thresholds of about 0.9V or 1.0V.
A problem, however, may arise when driving the interconnection from a "higher high" to a "lower high" if the second driver's impedance is too low. If the impedance of the driver that is trying to establish the 1.5V uplevel (in the above example) is too low, the signal on the wire will significantly undershoot the 1.5V level enough to make a designated receiver erroneously switch, or at the minimum, reduce the signal margin at the receiver below safe limits. Simulations have shown that a driver, with an uplevel of 1.5V minus a supply tolerance, in its lower impedance mode, e.g., 20 ohms, would cause the signal received at a receiver to drop to about 1.0V. On the other hand, the same driver in a higher impedance mode, e.g., 40 ohms, would provide a 1.25V signal at the receiver. An even higher impedance driver will drive the net at an even higher voltage uplevel.
Accordingly, what is needed in the art is an improved driver circuit that overcomes or mitigates the above discussed limitations.